Magnetic memory cells are memory cells that store information in the orientation of the magnetization of a ferromagnetic region. These magnetic memory cells are nonvolatile and can hold stored information for long periods of time. Magnetic memory cells that use a magnetic state to alter the electrical resistance of the materials near the ferromagnetic region are collectively known as magnetoresistive (MR) memory cells. An array of magnetic memory cells is often called magnetic RAM or MRAM (magnetic random access memory). MRAM arrays include an array of magnetic memory cells positioned at the intersections of wordlines and bitlines. Each cell includes a magnetically changeable or free region and a proximate reference region arranged into a magnetic tunnel junction (“MTJ”) device. The principle underlying storage of data in such cells is the ability to change the relative orientation of the magnetization of the free and reference regions by changing the direction of magnetization along the easy axis (“EA”) of the free region, and the ability to thereafter read this relative orientation difference. More particularly, MRAM cells are written to by reversing the free region magnetization using applied electrical, and resultant magnetic stimuli via their respective bitline and wordline, and are later read by measuring the resultant tunneling resistance between their bitlines and wordlines. The tunneling resistance assumes one of two values depending on the relative orientation of the magnetization of the free region with respect to the reference region. If the free region is modeled as a simple elemental magnet having a direction of magnetization which is free to rotate but with a strong preference for aligning in either direction along its easy axis (+EA or −EA), and if the reference region is, for example, a similar elemental magnet but having a direction of magnetization fixed in the +EA direction, then two states (and therefore the two possible tunneling resistance values) are defined for the cell: aligned (+EA/+EA) and anti-aligned (−EA/+EA). These two states may be used to represent a logical “1” or “0” for typical binary processing applications.
Coercivity refers to the magnetic field strength required to alter the orientation of the magnetization in a magnetic memory cell. A write current is typically used to provide the required magnetic field needed to alter the orientation of the magnetization of the magnetic memory cell. In a MRAM device, the bits should all have approximately the same coercivity so that they can all be altered using a write current and corresponding magnetic field of approximately the same strength. However, changes in the environment of an MRAM chip, or in the chip itself, may cause variations in the switching parameters of magnetic memory cells. One way to compensate for these changes in environmental conditions is to alter the magnetization of the magnetic memory cell with a write current that is larger than the minimum current needed to alter the orientation of the magnetization. Nevertheless, if the changes in the operating environment are serious enough, the write margin of the write current may be reduced to the point that unacceptable write errors occur.
A variety of other measures have been used to minimize the likelihood of write errors due to changes in environmental conditions. For example, magnetic shielding may be employed to reduce the effects of changes in the surrounding magnetic field on the magnetic memory cells. In a similar fashion, error causing temperature changes can be minimized by housing the magnetic memory cells in a temperature controlled environment. Although these measures are helpful, they suffer from a number of drawbacks. For instance, magnetically shielding the memory cells may fail to achieve the desired results if the shield material becomes saturated or the external fields are lower in amplitude than the coercive field of the shield. In addition, providing a temperature controlled environment may be unduly expensive or impractical in certain situations.
Therefore, there is a need for an improved method and apparatus for compensating for changes in environmental factors that may cause write errors when writing information to a magnetic memory array.